1. Field of the Invention
The present invention relates to a method of manufacturing superconductive products, particularly those of high critical temperatures (Tc) and improved critical current densities (Jc).
2. Description of the Related Art
Nb-Ti, Nb-Zr, Nb-Ti-Zr, Nb.sub.3 Sn, V.sub.3 Ga and the like have been practically used as superconductive matters. These matters can be easily prepared by plastic processing raw materials which have been compounded with a stabilizing metal such as Cu and Al. It has been well known that compounds such as Nb.sub.3 Sn are diffused at the final process according to the bronze method.
However, these matters have critical temperatures (Tc) lower than about 20.degree. K. and need liquid helium, which is expensive and extremely low in temperature, as their cooling medium.
Superconductive matters of the ceramic or nonmetal type such as oxides having critical temperatures higher than the boiling point of cheap liquid hydrogen or nitrogen have been recently provided. These are stratified compounds of the K.sub.2 NiF.sub.4 or three-layer perovskite type in the group of Ln--A--Cu--O wherein Ln represents Y, Sc or other rare earth elements and A denotes alkaline earth elements such as Ba and Sr. They are, for example, YBa.sub.2 Cu.sub.3 O.sub.7 --.sub..delta. ErBa.sub.2 Cu.sub.3 O.sub.7 --.sub..delta. YBaSrCu.sub.3 O.sub.7 --.sub..delta. (La.sub.1-x Sr.sub.x).sub.2 CuO.sub.4 and the like and include even those where oxgen is replaced by fluorine, sulfur or the like.
The superconductive matters of the ceramic type can become superconductive at high temperature (Tc) and are expected to be used for power transmission lines of large capacity, magnets of high magnetic field and so on. However, it is extremely difficult to process the ceramic superconductive matters, as compared with those of the metal type such as Nb, and it has been therefore proposed that powder of the ceramic superconductive matters which has been previously subjected to the burning treatment or which is not burned yet is used as raw material, and that this raw material powder is deposited on a base of desired form or molded, metal covered and elongated to a line product. These line products are further processed to a twisted line or coil, if necessary.
In the case of the superconductive matters of the oxide type such as YBa.sub.2 Cu.sub.3 O.sub.7 --.sub..delta., therefore, raw material powder of oxide and carbonate, for example, is preburned to provide a compounded oxide, which is powdered, then made to fill a metal sheath, compressed by the rubber press to raise its density, and then processed to a line product of desired diameter. These line products are twisted together to a twisted line or processed to a coil, if necessary, which is further sintered in an air or oxygen atmosphere.
Even if the powder of the superconductive matters is made to fill the metal sheath and processed to the line product according to the conventional method of manufacturing the ceramic superconductive products, the density of the powder filling the metal sheath is low, ranging about 70%, and when this powder made to fill the metal sheath like this is processed to a twisted line or cable and then sintered, its volume is contracted more than 20%. Clearance is thus formed between the powder and the metal sheath and this causes the thus-made products to be inferior in their superconductivity.
Even when the superconductive matter is elongated to a line product, it is difficult to arrange its crystals in a direction because it is powder made to fill the metal sheath. This also make it impossible to provide superconductive products having excellent superconductivity.